Doning the outfit of a previous incarnation because it was the only one that would satisfy the UNIT commander, the David Segal Doctor is firmly in control of the situation. He has a vast knowledge of the universe and is aptly able to put that knowledge to use to stand up to and defeat anyone who would threaten the innocent. Yet sometimes the Doctor's overconfidence works against him, putting himself and those around him into situations that the more pragmatic might have avoided.
There is a friendly, down-to-earth nature to this Doctor. Though a Lord of Time and a Citizen of the Universe, those who get to know him can easily find themselves fishing from a local pond, passing away the hours or even days in casual conversation, while the next week flying off to the remotest parts of the universe in the farthest future. Time and Space are the natural elements of this incarnation.